1. Field of the invention
This invention concerns a sheet-fed press in which the sheet being fed is stabilized. More specifically, it concerns a sheet guide unit in the sheet-fed press. The sheet guide unit with a curved sheet guide surface is provided under the intermediate cylinder or the delivery cylinder, and it is separated from those cylinders by a small sheet guide space which serves as a guide for feeding the sheet.
2. Description of the Related Art
Multiple-color sheet-fed presses which employ a series of printers each of which prints a different color ink are well known in the prior art. As can be seen in FIG. 5, the basic structural elements of such presses are feeder unit A, which consists of feeder device 39; printer unit B, which has four printers, 132a, 132b, 132c and 132d, arrayed in tandem to print cyan, magenta, yellow and black; and delivery unit C, here paper delivery unit 04.
In multiple-color sheet-fed presses with this configuration, a sucker unit with an inlet for sheets 11, which are piled on table 141 of the feed unit 39, separates a single sheet and transports it on conveyor 120. Swing gripper 121a delivers the sheet to intermediate cylinder 121b of printer 132a. The sheet is fed between blanket cylinder 22a and impression cylinder 23a, and the first color is printed.
Once the first color has been printed, the sheet is fed out between the blanket cylinder 22a and impression cylinder 23a and taken up by intermediate cylinder 27a of the second printer 132b. From the intermediate cylinder 27a, the sheet is delivered to impression cylinder 23b. The next process, the printing of the second color, is executed by blanket cylinder 22b and impression cylinder 23b. 
The subsequent colors are printed one after the other. When sheet 11 is fed out between blanket cylinder 22d and impression cylinder 23d, which perform the final-stage printing, it is pulled onto delivery cylinder 35 of delivery unit C. From delivery cylinder 35, the now completely printed sheet 11 is taken onto chain conveyor 124 and transported to delivery unit 04, where it is added to the stack on table 40 of the unit 04.
Generally, the sheets 11 which are printed in a sheet-fed press are of a thickness which ranges from 0.04 m/m to 0.8 m/m. At times, high-rigidity sheets of metal plate or synthetic resin might also be printed. As the sheet is fed from printer 132a to printer 132b to print the various colors, various mishaps may occur. A thin sheet of paper will generally have low rigidity, and its rear portion will tend to flap. A thicker sheet of paper or sheet metal will have high rigidity, and its reaction force (stability) against the centrifugal force of rotation and its own curvature will cause its rear portion to separate from impression cylinder 23, and collide with the sheet guide unit 1 below the cylinder resulting a paper rebounding.
When the paper flaps or rebounds in this way, the print may be smudged or the paper folded or torn. This phenomenon is a significant cause of a reduction in print quality. Two typical methods employed to counteract this problem are to use a skeleton cylinder or a drum cylinder for the intermediate cylinder 27. This allows the most appropriate scheme to be used for the rigidity of whatever sheet is being printed.
The example shown in FIG. 6(A) is a skeleton-type intermediate cylinder 27, which is used primarily when printing thicker sheets of paper. One of these skeleton cylinders 27 is placed on each side of each printer 132. Each skeleton cylinder consists of a pair of rotors (arms) 271 which rotate on axis 270. Each arm 271 has a series of pawls 29 on its shaft 272 (see FIG. 7(A)) running from the end of arm 271 to the end of arm 271 on the opposite side of the shaft. The distinguishing feature of the skeleton cylinder 27 is that the area of the cylinder which comes in contact with impression cylinder 23 when the paper passes between them is extremely small. The sheet 100 which is being rotated forward is allowed to bend beyond point P where it comes into contact with pawls 29. In other words, the point of contact P becomes the point of action. By lengthening the distance from this point to the end of sheet 100, we reduce the reactive force exerted by the sheet in its attempt to return to its original shape.
As a result, we reduce the amount of rebounding at the end of the sheet which strikes sheet guide unit 1xe2x80x2, the curved guide which conforms to the hypothetical circumference of the lower portion of skeleton-type intermediate cylinder 27. This scheme minimizes tears and folds; but on the other hand, because this sort of skeleton cylinder 27 provides a larger region in which the end of sheet 100 is free, a thin sheet will have more opportunity to flap.
The example shown in FIG. 6(B) is drum cylinder-type intermediate cylinder 27xe2x80x2, which is used primarily for thinner sheets of paper. This sort of drum cylinder 27xe2x80x2 has a number of pawls 29 in two places along the circumference of a roller which rotates on axis 270.
The feature which distinguishes drum cylinder 27xe2x80x2 is that the amount of its surface area which comes in contact with impression cylinder 23 as sheet 100 is fed between them is maximized. Because the portion of sheet 100 which is beyond pawls 29 is guided along the circumference of the drum cylinder (27xe2x80x2), this scheme makes it very difficult for the end of the sheet to flap, so it minimizes doubling, tearing and other defects resulting from the end of the sheet wrinkling or flapping. However, when this sort of drum cylinder 27xe2x80x2 is used to convey thicker varieties of paper, the fact that there is very little area where the end of the sheet is free will result in significant rebounding.
In recent years, as print quality has improved, there has been a tendency to use the skeleton cylinders even for thinner papers. To keep thin sheets from flapping, a sheet guide unit 1 is provided which has a sheet guide surface 1d following the contour of the lower portion of intermediate cylinder 27 (or 27xe2x80x2) and delivery unit 35 (hereafter referred to as the intermediate cylinder). In order to address the problems in this sort of sheet-fed press, a sheet guide unit is provided in which specifically pressurized air is blown through a number of vents in the sheet guide unit into the space between intermediate cylinder 27 and surface 1d of the sheet guide unit. This air is blown along the bottom of sheet 11 as it passes through the space along sheet guide surface 1d. Because of the Bernoulli effect, the air blown through the vents causes the sheet 11 to be suspended.
One such sheet guide unit is described in Japanese Patent Publication (Kokai) Hei 10-109404. We shall explain the relevant technology with reference to FIG. 7. The sheet guide unit, which runs along the circumference of skeleton-type intermediate cylinder 27 or delivery cylinder 35, both of which are studded with pawls 29, consists of air ducts 06. On the upper surface of the air ducts 06 are numerous air vents 4a and 4b. The vents 4a and 4b face in opposite directions and are located on either side of the center of the intermediate cylinder 27 or of delivery cylinder 35. The vents distribute the air toward the outer edges of the intermediate cylinder 27. The vents 4a and 4b produce two streams of air which originate at the vents and continue to move in the directions determined by the vents. These air streams keep the sheet of paper suspended at a specified height, thus stabilizing the travel of the sheet.
In the prior art technique, then, air is blown through a space between sheet guide surface id and the intermediate cylinder underneath sheet 11. The sheet is caught on pawls 29 of skeleton-type intermediate cylinder 27, the type of cylinder used for thicker papers. The air is blown into the space from ducts 06 below the guide surface through the air vents 4a and 4b. More specifically, as can be seen in FIG. 7(B), streams of air are blown toward both sides of intermediate cylinder 27 through vents which face away from each other on either side of the middle of the cylinder 27. These streams of air create a difference in the rate of the airflow above and below the sheet, thus producing the Bernoulli effect. The sheet 11 which is being conveyed along the surface of the intermediate cylinder 27 is drawn toward surface 1d of the sheet guide unit and suspended slightly above it as it is conveyed, before being taken up by the next impression cylinder 23.
This sheet guide unit has an aspiration duct 3xe2x80x2 which exhausts at the outlet end of guide surface 1d. On either side of guide surface 1d of duct 2xe2x80x2 are air vents 4a and 4b. The aspiration duct 3xe2x80x2 is connected to duct 2xe2x80x2, which is in the interior of the unit, via fans 51.
Because the duct 3xe2x80x2 is provided on the outlet end of the guide surface, the air which is blown across the width of the sheet along surface 1d of the sheet guide unit will be drawn into aspiration duct 3xe2x80x2 by the action of fans 51. The air directed by fans 51 is drawn into aspiration duct 3xe2x80x2 and redirected by duct 2xe2x80x2 toward vents 4a and 4b. 
However, the prior art technology suffers from the following problems.
In the sheet guide unit 1xe2x80x2, aspiration duct 3xe2x80x2 and duct 2xe2x80x2 are connected, so the volume of air driven by fans 51 and the volume drawn into the aspiration duct must be equal. However, if the same volume of air is drawn into the aspiration duct, not all of the air flowing over surface 1d of the sheet guide unit can be drawn in. More specifically, the sheet guide unity 1xe2x80x2 is mounted inside two sets of frames 011, which support the cylinders of the sheet-fed press. From the aspiration duct 3xe2x80x2, the excess air will end up escaping into the press mechanism. Some of the air blown out through vents 4, in other words, will not be drawn into the duct. After the air is used to draw sheet 11 toward the sheet guide unit, this air will collide with frame 011 and cause undesirable turbulence in the press mechanism. If a thinner paper is being printed, this may cause its lateral edges to flutter.
To address this problem, the prior art design shown in FIG. 8 isolates aspiration ducts 3xe2x80x2 and propulsion ducts 2xe2x80x2 by interposing partitions 52. Instead of a fan, it employs a pump 13xe2x80x2 to drive a larger volume of air.
However, with this configuration, the volume of air propelled by the pump and the volume aspirated will still be equal, just as in FIG. 7. With this prior art design, the air stream propelled from the nozzle of the guide surface will be moving at a high velocity (approximately 20 to 30 m/s), so it will have a high inertial force. Below the nozzle, a turbulent boundary layer will begin to form, and the flow itself will become thicker and move away from the surface of the sheet guide unit.
With this prior art design, then, the recovery of the air flow from both sides of the sheet guide unit into the chamber provided on each side will be inefficient. The unrecovered air will collide with the frame, causing turbulence within the frame of the press mechanism. This turbulence will disrupt the flow in the upstream segment of the sheet guide space. If a thinner stock is being printed, the end of the sheet is very likely to flap or flutter. If the intermediate cylinder is a skeleton cylinder, conveying a thinner paper becomes extremely problematic.
In view of these problems in the prior art, the object of this invention is to provide a sheet guide unit for a sheet-fed press which prevent the sheet from flapping or fluttering, and would allow sheets of thinner paper to be conveyed smoothly even when a skeleton cylinder, which is better suited to thicker papers. The sheet guide unit according to this invention has a sheet guide space in which a sheet can pass. The sheet guide space is provided between a printing cylinder and a sheet guide unit. Air is blown through vents on the sheet guide unit into the sheet guide space. The sheet guide unit for such a press can prevent the air streams flowing through the sheet guide space and exiting from both ends of the sheet guide unit from colliding with the frame and causing turbulence.
To solve this object, the sheet guide unit according to this invention is configured as follows. This sheet guide unit is provided below a printing cylinder, such as an intermediate cylinder and a delivery cylinder of sheet-fed press, below which is fashioned a curved sheet guide surface separated by a small sheet guide space. The sheet guide unit has air supply chambers which are behind the sheet guide surface, and numerous air vents which vent air from the air supply chambers into the sheet guide space. The air vents face away from each other toward the sides of the cylinder on either side of its center line. They vent air along the surface of the sheet guide unit along the width of the cylinder. The difference in the velocity of the air flow above and below the sheet being conveyed by the rotation of the cylinder then causes the sheet to be drawn toward the surface of the sheet guide unit and suspended slightly above it as it is conveyed.
The sheet guide unit is characterized by the following configuration. At least a pair of air aspiration chambers would be provided adjacent to the air supply chambers on the outer sides of the cylinder at the outlets of the sheet guide unit. The outlet ends of the sheet guide surface would be extended, and the extended portions would lead into the air aspiration chambers so that they could serve as guide fins to direct the air into the chambers. The volume of air drawn into the aspiration chambers on either side of the cylinder would be larger than the volume of air blown into the aspiration chambers. This would create a negative pressure in the vicinity of the ends of the sheet guide surface.
The actual design of the guide fin should be as follows. Its cross section should form an angle xcex1 of 20 to 40xc2x0 with respect to the sheet guide surface of the sheet guide unit. Ideally, it should be a straight fin set at an angle xcex1 a of approximately 30xc2x0. Alternatively, the fin may have a curved cross section so that its curved surface leads into the aspiration chamber.
The specific relationship between the volume of air blown into the chambers and the volume drawn into the chambers should be as follows. Exhaust. pumps should be connected to the aspiration chambers, and supply pumps should be connected to the supply chambers. These may be regulated so that the volume of air exhausted by the exhaust pumps is larger than the volume supplied by the supply pumps. Alternatively, recirculation paths may be created by installing recirculation pumps between the aspiration and supply chambers. In this case, escape valves should be provided between the outlets of the recirculation pumps and the air supply chambers to allow a portion of the air to escape from the recirculation paths.
With this invention, then, a negative pressure is created on the outlet ends of the sheet guide unit on both sides of the printing cylinder. The ends of the sheet guide unit are extended, and the extended portions lead into the air aspiration chambers so that they can serve as fins to direct the air into the chambers. Thus even when the air stream flowing along the surface of the sheet guide unit is moving at a high velocity, all of the air directed to the outlets of the sheet guide unit will flow along the guide fins and be drawn into the chambers.
As a result, the air stream flowing through the sheet guide space cannot overflow and collide with the frame, causing thinner papers to flap. In other words, this scheme allows us to minimize turbulence in the air stream throughout the entire sheet guide space. Even when a skeleton cylinder is used, thinner papers can be conveyed without problems.
The air is sucked efficiently into the aspiration chambers; and the negative pressure at the ends of the sheet guide unit has the effect of reducing the thickness of the boundary layer on the sheet guide surface of the sheet guide unit near the ends of the guide. This prevents eddies from forming, thus making it easier to draw the sheet toward the surface of the sheet guide unit when a thinner paper is being printed. It will prevent thinner papers from flapping or buckling.
With this invention, then, the effect of the negative pressure and the guide fins prevent eddies from forming at the ends of the sheet guide surface. This insures that the flow of air through the entire sheet guide space will be virtually free of turbulence. The turbulent boundary layer under the sheet due to the air stream will be thinner, so the sheet is less likely to flap or flutter, but will be conveyed smoothly through the sheet guide space.
So simply by adding guide fins and increasing the volume of air drawn into a pair of chambers, i.e., through a simple and inexpensive design, we can prevent thinner sheets from flapping or buckling when a skeleton cylinder is used and enable them to be conveyed smoothly.
Because the guide fins have the particular configuration described above, the air stream will flow along the surface of the fins without hindrance. The flow is less likely to burble from the surface of the guide, and turbulence in the sheet guide space will be kept to a minimum, thus stabilizing the flow.
The negative pressure at the ends of the guide has the effect of suppressing the formation of a turbulent boundary layer over the sheet guide unit. The layer which does form will be thinner, and the flow will be more stable. The Bernoulli effect will be maximized in the sheet guide space, allowing the sheet to be conveyed more smoothly. Although the same effect may be obtained by connecting a number of independent pumps of different capacities, it may also be obtained by installing an escape valve to exhaust a portion of the air on the forward side of the pump which recirculates air along the path between the aspiration and supply chambers. Since the latter scheme can be implemented using only one recirculation pump, it would reduce the cost of equipment to choose this option.
By adjusting the escape valve, we can control both the rate of flow and the pressure of the air flowing through the recirculation pipe. This valve makes it easy to adjust the Bernoulli effect in the sheet guide space.